Rework day 1

Simplify part 2 a lot, by not actually computing the sums because they
do not matter, only the changes do. Also eliminate the allocation
overhead while parsing line-by-line input.

Fixes the existing clippy error because the offending line no longer
exists.

2021/src/lib.rs

@@ -2,6 +2,7 @@ use std::io::Read;
 
 type Solution = fn(&mut dyn Read) -> String;
 
+mod common;
 mod day01;
 mod day02;
 mod day03;